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KentuckyFC writes "Last year, a group of theoretical physicists suggested a bizarre experiment based on a quantum phenomenon known as weak measurement. Unlike ordinary measurements that always change the state of a quantum object, a weak measurement extracts such a small amount of information that it leaves the quantum state intact. For example, a weak measurement can detect the presence of a photon by the deflection it causes when it bounces off a mirror. However, this does not change the photon's quantum state. The new idea was to make two weak measurements on a quantum system that is in a superposition of states, the goal being to separate the location of this quantum system from its properties, like a Cheshire cat. Now a group of experimentalists say they've observed a quantum Cheshire cat for the first time in an experiment involving neutrons. They passed a beam of neutrons through a magnetic field to align their spins and then sent them through an interferometer in which the neutrons pass down both arms of the experiment at the same time. They then used weak measurements to locate the neutrons in one arm while measuring their magnetic properties in the other. Voila! A quantum Cheshire cat."

"a weak measurement extracts such a small amount of information that it leaves the quantum state intact."

That's not correct description -- the quantum state is changed, albeit less than with projective measurement. The paper itself [arxiv.org] calls it in the abstract "minimal disturbing" measurement, not the "non-disturbing" measurement.

"The paradox arises when the team carried out two weak measurements. The first found the presence of neutrons in one arm while the second noted their magnetic properties in the other arm. “The neutrons behave as if particle and magnetic property are spatially separated while travelling through the interferometer,” they say. In other words, they observed a quantum Cheshire cat."

Per the peer review: "Twas brillig, and the slithy toves. Did gyre and gimble in the wabe: All mimsy were the borogoves, And the mome raths outgrabe."

Its because the CC's attributes can be in one place while its body is somewhere else... after all, it can be between the state of abnormal and nothing: the last thing to fade is the smile (not the teeth and lips, but the smile) and it can interact without in fact being there.